Surface complexation of arsenic(V) to iron(III) (hydr)oxides: structural mechanism from ab initio molecular geometries and EXAFS spectroscopy
Surface complexation of arsenic(V) to iron(III) (hydr)oxides: structural mechanism from ab initio molecular geometries and EXAFS spectroscopy
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DOI：
10.1016/s0016-7037(03)00237-0
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发表时间：
2003-11
影响因子：
5
通讯作者：
D. Sherman;S. Randall
中科院分区：
文献类型：
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作者：
D. Sherman;S. Randall
砷（V）以砷酸盐（AsO 4）3−离子及其共轭酸的形式，强烈吸附于氧化铁（α-Fe 2 O3）、氢氧化物（α-，γ-FeOOH）和结晶度低的水铁矿（水合氧化铁）上。砷酸盐与氧化铁氢氧化物表面络合的机制尚未完全了解。有明确的证据表明内球络合，但表面络合物的性质是有争议的。AsO 4四面体和FeO 6表面多面体之间可能的表面复合物包括双齿共角（2C）、双齿共边（2 E）和单齿共角（1V）。采用密度泛函理论方法对Fe 2（OH）2（H2O）nAsO 2（OH）23+和Fe 2（OH）2（H2O）nAsO 4+簇进行了计算，预测了AsO 4-FeOOH表面复合物的相对能量和几何构型.双齿角共享复合物被预测为基本上（55千焦/摩尔）更有利于积极的假设边缘共享双齿复合物。单齿角共享（1V）复合物非常不稳定。我们测量了0.3 wt. %（AsO 4）3−吸附在赤铁矿（α-Fe 2 O3）、针铁矿（α-FeOOH）、纤铁矿（γ-FeOOH）和水铁矿上，并直接与EXAFS多重散射拟合。EXAFS谱的相移校正傅里叶变换显示出2.85和3.26 nm附近的峰，这些峰被以前的研究者归因于2 E和2C络合物。然而，我们发现，2.85欧姆附近的峰值似乎是由于As-O-O-As多次散射，而不是从As-Fe背散射。所观察到的3.26 μ As-Fe距离与双齿角共享表面（2C）复合物的预测一致。我们没有发现单齿（1V）复合物的证据，这与预测的高能量的复合物。
Arsenic(V), as the arsenate (AsO4)3−ion and its conjugate acids, is strongly sorbed to iron(III) oxides (α-Fe2O3), oxide hydroxides (α–,γ–FeOOH) and poorly crystalline ferrihydrite (hydrous ferric oxide). The mechanism by which arsenate complexes with iron oxide hydroxide surfaces is not fully understood. There is clear evidence for inner sphere complexation but the nature of the surface complexes is controversial. Possible surface complexes between AsO4tetrahedra and surface FeO6polyhedra include bidentate corner-sharing (2C), bidentate edge-sharing (2E) and monodentate corner-sharing (1V). We predicted the relative energies and geometries of AsO4-FeOOH surface complexes using density functional theory calculations on analogue Fe2(OH)2(H2O)nAsO2(OH)23+and Fe2(OH)2(H2O)nAsO4+clusters. The bidentate corner-sharing complex is predicted to be substantially (55 kJ/mole) more favored energetically over the hypothetical edge-sharing bidentate complex. The monodentate corner-sharing (1V) complex is very unstable. We measured EXAFS spectra of 0.3 wt. % (AsO4)3−sorbed to hematite (α-Fe2O3), goethite(α–FeOOH), lepidocrocite(γ–FeOOH) and ferrihydrite and fit the EXAFS directly with multiple scattering. The phase-shift-corrected Fourier transforms of the EXAFS spectra show peaks near 2.85 and 3.26 Å that have been attributed by previous investigators to result from2E and2C complexes. However, we show that the peak near 2.85 Å appears to result from As-O-O-As multiple scattering and not from As-Fe backscatter. The observed 3.26 Å As-Fe distance agrees with that predicted for the bidentate corner-sharing surface (2C) complex. We find no evidence for monodentate (1V) complexes; this agrees with the predicted high energies of such complexes.